The Permian-Triassic deep marine ultra-deep high-temperature and high-pressure water-bearing gas reservoirs in Qixia Formation and L Gas Field, Y Block, western Sichuan, are characterized by difficult and high-cost pressure measurement operations. In actual production, there is a large demand for conversion of bottom hole flowing pressure by model. In order to improve the calculation accuracy of bottom hole flowing pressure, the density, viscosity, surface tension and other parameters of formation water under high temperature and high pressure conditions of the gas reservoir are obtained through the experiment of physical parameters of formation water under high temperature and high pressure. On this basis, the empirical formula of formation water physical parameters in the unsteady heat transfer wellbore temperature model of ultra-deep high temperature and high pressure gas reservoir is modified for the first time. The flow pressure calculation of 12 wells in 6 wells in the field shows that the calculation accuracy of the high temperature and high pressure wellbore pressure prediction software after using the modified fluid parameters is increased from 97.51% to 98.76%, which is significantly higher than that of the commercial software 94.12%. The research results meet the accuracy requirements of single well productivity and dynamic reserve evaluation on site.